More than a dozen juvenile advocacy groups have filed a legal brief in support of freeing Cyntoia Brown, a woman whose life sentence for murder at the age of 16 has attracted global attention.

Brown, now 29, was convicted in the 2004 murder and robbery of Johnny Allen, a 43-year old Nashville real estate agent. Allen was a stranger to Brown when he picked her up at a Nashville fast food restaurant and brought her to his home, where Brown said she shot him as he lay naked beside her because she feared for her life. At the time, Brown claimed she had been forced into prostitution by her then-boyfriend.

On Jan. 9, Brown's attorneys filed an appeal of her conviction and sentence with the Sixth Circuit Court of Appeals. It is that appeal that 13 national juvenile justice advocacy groups have joined to file a supportive legal brief, known as an amicus brief.

Other groups joining the brief include the Southern Poverty Law Center, The Sentencing Project, the Center for Wrongful Convictions of Youth and others.

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Inmate Cyntoia Brown (right) of the Tennessee Prison for Women gets a hug from a Lipscomb University faculty member after delivering a commencement address before she received her Associate Degrees from Lipscomb University Friday Dec. 18, 2015, in Nashville, Tenn.(Photo: George Walker IV / The Tennessean)

"Cyntoia's sentence is wholly disproportionate for a 16-year-old girl, and therefore unconstitutional," said Marsha Levick, deputy director and chief counsel for Juvenile Law Center, which also joined the amicus brief.

The amicus brief notes that U.S. Supreme Court decisions have said in most cases a sentence of life imprisonment for juveniles is unconstitutional. The Tennessee Court of Criminal Appeals has already ruled against that argument in Brown's case, noting that her sentence is not entirely for life, since she will have the possibility of parole after 51 years.

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"With a life sentence and a chance for parole only after 51 years of incarceration, the court has sentenced Cyntoia to die in prison," Levick said. "Moreover this sentence was mandatory and thus imposed without any consideration of Cyntoia's background, childhood, or other distinctive developmental attributes."

Legal advocates for Brown argued that Brown didn't have the mental state necessary to be culpable for murder.

Experts who testified in Brown's sentencing hearing noted she may have suffered from fetal alcohol syndrome disorder in utero, impairing her mental abilities.

"This was a seriously mentally impaired girl, subject to the immaturity and impulsiveness of all juveniles but to a much greater degree, who had been abandoned by her parents and whose only refuge was a pimp who sexually and physically abused her," Brown's lawyers' appeal said.

A spokeswoman for the Attorney General, which will defend the case, said she was unable to comment on ongoing litigation.

Organizations filing brief in support of Cyntoia Brown

Juvenile Law Center, Southern Poverty Law Center, Center on Wrongful Convictions of Youth, Bluhm Legal Clinic, Northwestern Pritzker School of Law, Children and Family Justice Center, The Sentencing Project, Campaign for Fair Sentencing of Youth, The Phillips Black Project, Massachusetts General Hospital Center for Law, Brain and Behavior, and Robert F. Kennedy Human Rights.

Reach Anita Wadhwani at awadhwani@tennessean.com; 615-259-8092 or on Twitter @AnitaWadhwani.